This invention relates to an improved method for driving a matrix type display panel in which capacitive display cells are arranged in the form of a matrix, and more specifically to a new method for driving a display panel such as a thin film electroluminescence (EL) display device in such a manner that fluctuations in the brightness of the light that is emitted, caused by the influence of electrode resistance, can be alleviated.
As a matrix display device wherein capacitive display cells are arranged in the form of a matrix, a display panel having the structure of a plurality of scanning electrodes and a plurality of data electrodes are arranged in orthogonal directions opposite each other on both sides of a display medium such as an EL material or a discharge gas, with each plurality of electrodes being on an insulating layer, is very popular. As an example, an AC-driven type of thin film EL display panel generally provides a multi-layer thin film structure as shown in FIG. 1(A). Namely, the panel 10 has a structure such that a translucent data electrode 2 is provided on a translucent glass substrate 1, an EL layer 4 such as ZnS:Mn sandwiched by the insulating layers 3, 5 from both sides is placed thereon, and a metal scanning electrode 6 like Al is provided on the upper insulation layer 5. The data electrode 2 and scanning electrode 6 are arranged in a matrix in mutually orthogonal directions, the display cells 7 are defined at the intersection points of the opposing electrodes and the selected display cells emit light upon receiving a combined voltage of a scanning pulse selectively applied to both electrodes and a data pulse. For such a panel structure, the following refresh drive method is employed. Namely, the entire surface is address-scanned on the lines on a time-shared basis with such a selection pulse and then the address points are caused to emit the light again by applying in common a refresh pulse with a polarity opposite to that of said selection pulse.
However, in case of an EL display panel of such a structure, the electrode resistance of the translucent electrodes on the substrate side to be used generally as the data electrodes 2 inevitably becomes higher than the electrode resistance of the metal scanning electrode 6 on the rear side. A translucent electrode is usually formed as a mixed vacuum-deposited film of tin oxide and indium oxide (ITO), and such a translucent electrode has a comparatively high resistance. Therefore an electrode resistance of about 20 k.OMEGA. results for an electrode length of 200 mm as in the case of forming a display panel of 1000.times.1000 cells with five electrodes per 1 mm and with each electrode having a width of 0.15 mm. As a result, when a panel having a large scale display area is to be driven, some different is generated in the rising waveforms of data pulses as between the display cell at the connection end of the data driver that is, in the (nearest cell) and the display cell furthest apart from the connection end (furthest cell), and accordingly the brightest of the emitted light is different.
Such a conventional problem is explained in more detail by referring to the panel model view of FIG. 1(B), the panel equalizing circuit of FIG. 2 and the driving voltage waveforms of FIGS. 3(a) to (h). In this case, each figures shows, as an example, that the display cell group related to the data electrode D.sub.1 is selected for light emission. In FIG. 1(B), 1 is the substrate, D.sub.1 .about.D.sub.1000 are translucent data electrodes, S.sub.1 .about.S.sub.1000 are metal scanning electrodes, S.sub.n is the display cell nearest to the data power supply (hereinafter referred to as the nearest cell within the panel), S.sub.f is the display cell furthest from the data power supply (hereinafter referred to as the furthest cell within the panel). Moreover, in FIG. 2, rd is the resistance value of the data electrode per cell, and CS is the cell capacitance. As is obvious from FIG. 2, the effective circuit of the panel electrode resistance and panel cell capacitance observed from the driving end of the data electrode D.sub.1 forms a ladder circuit, there is a large difference in the CR time constant at the ports nearer to and the ports further from the data power supply. Therefore, a data voltage pulse DP supplied from the data power supply to the data electrode D.sub.1 as shown in FIG. 3(a) is directly applied to the electrode nearest to said power supply as the half-selection voltage in the waveform shown in FIG. 3(b), but is applied to the furthest electrode as the half-selection voltage the rising edge of which is dulled as shown in FIG. 3(c). Therefore, a remarkable difference appears in the rising edges of the combined voltage at the full selection time when a half selection scanning voltage pulse SP is applied to the scanning electrodes S.sub.1 to S.sub.1000 as indicated in FIGS. 3(d) to (f). Namely, a difference occurs between the voltage waveform PS.sub.n as in the nearest cell Sn within the panel of FIG. 3(g) and the voltage waveform PS.sub.f of the furthest cell S.sub.f within the panel as shown in FIG. 3(h) The problem particularly arises in that the furthest cell S.sub.f cannot obtain a sufficient voltage for emitting the light and therefore brightness is lowered below that of the nearest cell S.sub.n, and accordingly the brightness of the light fluctuates over the entirety of the display cells.
On an actual EL display panel, the output terminals of the transparent electrodes are alternately placed along two opposing edges of the panel and connected to the drivers. Therefore, the nearest and furthest cells from the drivers alternate in a line along the edge of the panel, and the brightness nonuniformity of their display cells is obvious.
If electrode length and size are different, such a problem also occurs even when the same material is used for the electrodes (for example, a longer electrode has a higher electrode resistance).